The invention relates to a tire comprising at least one tread joined to two beads by means of two sidewalls and a carcass reinforcement, said tire being able to be used with or without an independent inner tube and being intended, after mounting on a standardized rim and after inflation, to be fitted on any vehicle capable of rolling.
The tires may be mounted on several types of standardized rim. Said rims may have frustoconical seats inclined relative to the axis of rotation, forming with said axis an angle which may be between 4xc2x0 and 16xc2x0. Among the latter, there are integral rims which are then referred to as hollow rims, semi-hollow rims or even drop-center rims, or rims made of several sections which are referred to as interim or xe2x80x9cadvancedxe2x80x9d rims. The rims in question may also have rim seats parallel to the axis of rotation and are generally referred to as flat-based.
The most frequently used rims, referred to as hollow or drop-center rims, comprise a mounting groove, the diameter of which is distinctly less than the nominal diameter of the rim. This rim groove diameter is considered by users to be too small, and it does not make it possible to select brake drums of dimensions suitable for effective braking of vehicles which are becoming ever more powerful.
It is furthermore highly desirable, whatever the reasons (reducing the pressures exerted by the tire on the ground, increasing the transverse stiffness of the tire, increasing the available internal space of the rim, lowering the center of gravity of certain types of machinery, to name but a few), to produce tires, the H/S form ratio of which is reduced, H being the height of the tire on its rim and S its maximum axial width.
The development of tires with reduced form ratios, more particularly intended to be fitted on heavy vehicle-type vehicles, such as lorries, buses, subway trains and tractor-trailer units, or of the very large heavy-vehicle type, such as loading, transport and earthmoving machinery used in construction, or aircraft machinery, is very delicate; in particular, the endurance of the beads becomes significantly less as the H/S ratio decreases, the inadequacies in endurance being found at the ends of upturns of the carcass reinforcement or the ends of reinforcement plies located in the beads of the tire, the structure of said beads being a conventional structure with, in each bead, a carcass reinforcement upturn and reinforcement plies, the meridian profiles of which are substantially parallel to the meridian profile of the carcass reinforcement in the zone of said beads. Furthermore, such form ratios, despite the possible reduction in the inflation pressures, involve a notable lack of comfort, which may be quite unbearable in the case of tires traveling long distances on relatively bumpy roads.
In order to overcome such disadvantages and to facilitate the production of tires, Application FR 94/14688 proposes a tire of form ratio of less than 0.8, intended to be mounted on a hollow rim or a drop-center rim, the seats of which are inclined relative to the axis of rotation by an angle which may be between 4xc2x0 and 16xc2x0. Said tire has a carcass reinforcement, surmounted radially by a crown reinforcement, the carcass reinforcement being composed of at least one ply of cords or cables and being turned up in each bead about an anchoring bead wire, passing from the outside to the inside to form an upturn, said upturn being reinforced by at least one additional reinforcement ply of metal cables, oriented by an angle of between 0xc2x0 and 20xc2x0 relative to the circumferential direction, the value 0xc2x0 being included in said range, and the meridian profile of said reinforced upturn, in the tire not mounted on its rim, being substantially parallel to the meridian profile of at least the portion of the seat located axially to the inside of the projection of the center of the circle circumscribed on the bead wire on said rim seat.
Although the production methods of a tire are currently very highly developed, the use of bead wires requires separate production and treatment of said bead wires (coating, for example), the transport and positioning of the same bead wires on a building drum or on a given support for manufacture of the bead, all operations which require time and multiple precautions, and which are therefore costly. Furthermore, since current tires are subjected to ever-increasing driving torques, the compromise between clamping on the rim which is necessary to avoid inopportune rotations and maximum clamping to permit mounting on the rim is difficult to control by the person skilled in the art in the current structures of tires with bead wires around which a carcass reinforcement is wound.
It has been discovered that the distribution of the contact pressures between the rim and the tire at the level of the respective rim and bead seats can be controlled and rendered as regular as possible in a very simple and reproducible manner while permitting an improvement in the resistance to unwinding of the carcass reinforcement in the event of a high degree of heating of the beads.
Thus, the tire according to the invention which is intended to be mounted on a standardized rim of axial width L, with rim flanges with rounded hooks, comprising a tread joined to two beads by two sidewalls and at least one carcass reinforcement formed of at least one ply of reinforcement elements, is characterized in that, viewed in meridian section, each bead is without a bead core or wire, said carcass reinforcement having in each bead a curved meridian profile which is formed axially and radially from the outside to the inside of at least one convex arc of a circle BC, possibly extended tangentially by a straight-line segment CD, to form a carcass reinforcement edge in the bead, located in an acute angle open axially and radially towards the inside, one of the edges of which is parallel to the axis of rotation, and of a value of at most 25xc2x0, said carcass reinforcement being reinforced in each bead at least radially to the outside and axially to the inside by a first continuous additional layer, of meridian profile substantially parallel to the meridian profile of said carcass reinforcement in the bead and comprising at least one section composed of at least one ply formed of inextensible reinforcement elements forming an angle of between xe2x88x922.5xc2x0 and +2.5xc2x0 with the circumferential direction, the radially upper end of said one section of the additional ply being distant from the axis of rotation by an amount at least equal to 0.96 times the distance between said axis and the point of the corresponding flange of the mounting rim farthest from said axis of rotation, and the axially inner end being distant from the equatorial plane by an amount at most 0.43 times the width L of the mounting rim.
xe2x80x9cRim flange, viewed in meridian sectionxe2x80x9d is to be understood to mean the assembly formed by the part substantially perpendicular to the axis of rotation extended, firstly, radially to the outside, by a circular part, forming what is sometimes referred to as the rim hook, and secondly, radially to the inside, by the arc of a circle connecting the axially outer end of the rim seat to said assembly. xe2x80x9cMeridian profile of a carcass reinforcement in a beadxe2x80x9d is to be understood to mean the meridian profile of the geometric center line of said reinforcement, viewed in meridian section, said profile being considered radially to the inside of a straight line parallel to the axis of rotation of the mounting rim and of the tire passing through the point of the rim hook, corresponding to the bead in question, the farthest from the axis of rotation.
xe2x80x9cInextensible reinforcement elementxe2x80x9d is to be understood to mean a cable or a monofilament having a relative elongation of at most 1.5% under a force of 20% of the breaking load. The additional layer will preferably be formed of metal cables or monofilaments, preferably of steel, in the case of tires of the xe2x80x9cheavy vehiclexe2x80x9d or xe2x80x9cconstructionxe2x80x9d type, but may advantageously be formed of textile elements, preferably of aromatic polyamide, in the case of tires for airplanes, agricultural tractors or passenger cars, for example.
A large number of rims and tires may be covered by the invention; the meridian profile of the assembly formed by the carcass reinforcement and the first additional layer, which profile is very closely linked to the profile of the carcass reinforcement, will, within a bead, be advantageously adapted to the meridian profile of the side of the rim on which said bead will be mounted. The meridian profile of the additional ply in effect does not differ from the meridian profile of the carcass reinforcement except by the possible variation of the rubber decoupling layer existing between the two plies, which explains why the two profiles are said to be substantially parallel.
Said meridian profile of the carcass reinforcement in a bead may advantageously be completed, radially and axially towards the inside, by a first, concave, arc of a circle extended tangentially by a second, convex, arc of a circle, said second arc of a circle possibly being extended tangentially by a straight-line segment.
The radially outer end of the first, concave, arc of a circle is radially distant from the axis of rotation by an amount equal to the radius of the circle which is the geometric locus of the points of the rim flange which are farthest from said axis. Said radially outer end is also the point of tangency with the convex meridian profile of the carcass reinforcement in the sidewall. The center of curvature of the first arc of a circle, relative to the two axes which are the trace of the equatorial plane and the axis of rotation, has the same coordinates as the center of curvature of the rim hook. As for the length of said first arc of a circle of the carcass reinforcement profile, it is at least zero, the meridian profile of the carcass reinforcement in the bead beginning directly with the second, convex, arc of a circle and at most equal to the length of an arc of a circle the radially lower end of which would be the point of tangency of said arc with a line perpendicular to the axis of rotation.
Said first arc of a circle is tangentially extended radially and axially to the inside by a second arc of a circle of a radius of curvature which may be between a minimum value of 5 mm and a maximum value equal to the radius of curvature of the meridian profile of the carcass reinforcement in the sidewall measured at the point of tangency between said profile and the first arc of a circle, said maximum value being the value of radius used preferably when the first arc of a circle has a length of zero.
The second arc of a circle is possibly tangentially extended radially and axially to the inside by a straight-line segment, the axially inner end of which is also the end of the meridian profile of the carcass reinforcement. The axially inner edge of the meridian profile is composed either of part of the second arc of a circle to which the straight-line segment is added, or of part of an arc of a circle. Said edge is the part of the meridian profile located in an angle defined by a half line forming a tangent to the second arc of a circle and forming an angle of 25xc2x0 with the axis of rotation and a half line parallel to said axis of rotation, the origin of which is precisely the point of tangency of the second arc of a circle with the half line oriented at 25xc2x0.
The first additional layer has an axially inner end which may be closer to or farther away from the equatorial plane than the axially inner end of the carcass reinforcement. Between its axially inner end and the vertex of the angle in which the edge of the meridian profile of the carcass reinforcement is located, the first additional layer may be separated from the carcass reinforcement by a low thickness of rubber mix, a total of the two respective thicknesses of the calendering layers of the two adjacent plies, whereas between said vertex and the radially upper end said thickness is between 1.0 and 1.8 times the thickness previously mentioned.
As described above, some tires may have a carcass reinforcement formed of textile elements. The end of the meridian profile of the carcass reinforcement can then not be merged with the end of said reinforcement, the latter being able to be turned up over the first additional layer.
In the case of tires having a high recommended inflation pressure, the first additional layer may be formed of two sections of several additional plies of inextensible reinforcement elements, superposed on one another, the lengths of which may or may not be the same. If the presence of inextensible elements is necessary for the entire first section of the additional layer located radially to the inside of a straight line parallel to the axis of rotation and located at a distance from said axis equal to 0.96 times the distance between said axis and the point of the corresponding flange of the mounting rim farthest from said axis of rotation, the possible second section of the additional layer extending the first section, located radially to the outside of said straight line, and therefore in the bead and even in the sidewall of the tire, may be formed of reinforcement elements which are either inextensible or have a certain extensibility or at least render said section extensible; thus continuous elements can be used for said part which are preferably metal ones and are said to be elastic, that is to say, having a relative elongation of at least 1.5% under a tensile force equal to 20% of the breaking load, or undulating or zigzag elements in the plane of the ply or plies forming the layer, or even of circumferentially discontinuous elements, each element having a circumferential length which may be between 0.1 and 1 times the circumferential length of the ply and the gaps between elements being meridianly offset relative to the gaps of the axially adjacent rows.
In the case of a high inflation pressure, the presence of a second continuous additional reinforcement layer may also prove very advantageous. Said second layer, axially to the outside and radially to the inside of at least the radially innermost and axially outermost ply of the carcass reinforcement, also has a meridian profile substantially parallel to the meridian profile of said carcass reinforcement in the bead. Said layer is formed of at least one ply of reinforcement elements. It may be continuous and homogenous over its entire length from its radially upper end which may be located radially to the outside, but preferably radially to the inside, of the straight line parallel to the axis of rotation of the rim and passing through the point of the rim flange, corresponding to the bead in question, which is farthest from the axis of rotation, until its axially inner end closest to the equatorial plane. It is then advantageously formed of inextensible cables, preferably metal ones, forming an angle of between xe2x88x922.5xc2x0 and +2.5xc2x0 with the circumferential direction. The second additional layer may also be non-homogenous over its width although continuous: it is then formed of two sections, or of three sections.
In the case in which its radially upper end is radially to the inside of the straight line parallel to the axis of rotation and passing through the point of the rim flange farthest from said axis of rotation, said second layer comprises a first section included between its radially upper end and a point located between the center of the second arc of a circle of the meridian profile of the carcass reinforcement in the bead and the vertex of the angle a in which the edge of the carcass reinforcement is located, and a second section, extending the first section axially to the inside and radially to the inside and included between said point and its radially lower end axially closest to the equatorial plane. The first section is formed of at least one ply of inextensible cables, while the second section is advantageously formed of at least one ply of reinforcement elements, said ply having the properties of being only very slightly compressible in the transverse direction and advantageously extensible in the circumferential direction, which, firstly, permits simple and reproducible control of the pressures exerted by the bead seat on the rim seat, the inventors having unexpectedly noted that said pressures were a function of the resistance to tension of the reinforcement elements constituting the ply (plies) of the second section of the second additional layer, and, secondly, facilitates the laying during manufacture of said additional layer.
In the event that the radially upper end of the second additional layer is radially to the outside of the straight line parallel to the axis of rotation and passing through the point of the rim flange farthest from said axis of rotation, the second layer may then comprise, in addition to the first and second sections previously described, a third section substantially located radially above the straight line parallel to the axis of rotation and passing through the point of the corresponding rim flange farthest from said axis of rotation.
The first section is formed of at least one ply of inextensible cables, preferably metal ones made of steel, and forming an angle between xe2x88x922.5xc2x0 and +2.5xc2x0 with the circumferential direction. The second section is advantageously formed of at least one ply of reinforcement elements, said ply having the properties of being only slightly compressible in the transverse direction and advantageously extensible in the circumferential direction, which, firstly, permits simple and reproducible control of the pressures exerted by the bead seat on the rim seat, the inventors having unexpectedly noted that said pressures were a function of the resistance to tension of the reinforcement elements constituting the ply (plies) of the second section of the second additional layer, and, secondly, facilitates the laying during manufacture of said additional layer. As for the third section, it is advantageously formed of at least one ply of reinforcement elements, said ply having the property of being extensible in the circumferential direction.
The ply (plies) of the second and third sections of the second additional layer may thus be formed of elastic, continuous, rectilinear reinforcement elements oriented circumferentially, said reinforcement elements preferably being metal or textile ones, like the elements referred to above and used for the second section of the first additional layer. They may be formed of undulating or zigzag reinforcement elements of circumferential average orientation, or of rows of discontinuous reinforcement elements, with the same characteristics as those referred to previously, said elements or rows of elements however in this case being parallel to each other and practically adjacent in the transverse direction. The ply (plies) of said second section may also be formed of metal reinforcement elements oriented at an angle of at least 80xc2x0 relative to the circumferential direction, said elements being circumferentially separated from each other by a distance of at least 0.2 mm. It may be advantageous in this latter case to provide one (several) width(s) of said ply (plies) sufficient to be able to turn it (them) up over the end of the carcass reinforcement. Furthermore, the laying of the second section of the second layer may be facilitated further if the ply (plies) of said section are formed of strips of several circumferential reinforcement elements, which strips are circumferentially discontinuous, the discontinuity gaps between strips forming with the circumferential direction an angle which is different from the angle formed with the same direction by the reinforcement elements of the carcass reinforcement, the difference being at least 10xc2x0.
In the event that the carcass reinforcement is formed of at most three plies, the second additional layer, whatever the number of plies and whatever the nature of the elements of which it is composed, is preferably located in its entirety axially to the outside and radially to the inside of the carcass reinforcement. In the event that the carcass reinforcement is formed of at least four plies, the plies forming the second additional layer are such that two of them may advantageously tightly surround a group which may be formed of one, two or three carcass reinforcement plies.